Martin Kuster

Reliability of estimating the room volume from a single room impulse response

The methods investigated for the room volume estimation are based on geometrical acoustics, eigenmode, and diffuse field models and no data other than the room impulse response are available. The measurements include several receiver positions in a total of 12 rooms of vastly different sizes and acoustic characteristics. The limitations in identifying the pivotal specular reflections of the geometrical acoustics model in measured room impulse responses are examined both theoretically and experimentally. The eigenmode method uses the theoretical expression for the Schroeder frequency and the difficulty of accurately estimating this frequency from the varying statistics of the room transfer function is highlighted. Reliable results are only obtained with the diffuse field model and a part of the observed variance in the experimental results is explained by theoretical expressions for the standard deviation of the reverberant sound pressure and the reverberation time. The limitations due to source and receiver directivity are discussed and a simple volume estimation method based on an approximate relationship with the reverberation time is also presented.

Acoustic imaging in enclosed spaces: Analysis of room geometry modifications on the impulse response

Sound propagation in enclosed spaces is characterized by reflections at the boundaries of the enclosure. Reflections can be wanted in the case when they support the direct sound or give a feeling of envelopment or they can be unwanted when they lead to echoes and colouration. When measuring multiple impulse responses in an enclosed space along an array the reflections can be mapped to the reflecting objects. Similar to seismic exploration, medical diagnostics, and underwater acoustics, an image of the reflecting objects is obtained in terms of reflected energy. The imaging process is based on inverse wave field extrapolation with the Kirchhoff–Helmholtz and Rayleigh integrals. The inverse of the imaging process recreates the measured impulse responses from the image and it allows one to remove or alter reflecting objects in the image and investigate their influence on the wave field in the enclosed space in a physically correct way. This can be verified by reimaging the altered wave field. Preliminary res...

Spatial correlation and coherence in reverberant acoustic fields : Extension to microphones with arbitrary first-order directivity

Spatial correlation and coherence functions in reverberant sound fields are relevant to the acoustics of enclosed spaces and related areas. Theoretical expressions for the spatial correlation and coherence functions between signals representing the pressure and/or the components of the particle velocity vector in a reverberant sound field are established in the literature and most of these have also been corroborated with measurements [F. Jacobsen, J. Acoust. Soc. Am. 108, 204-210 (2000)]. In the present paper, these expressions are generalized to microphones of first-order directivity, whereby the directivity can be expressed in terms of pressure and pressure gradient. It is shown that the resulting spatial correlation and coherence functions can be expressed in terms of the established spatial correlation and coherence functions. The derived theoretical expression for the spatial coherence function is validated with a modeled diffuse sound field. Further, it is compared with the experimental coherence obtained from the reverberant tails of room impulse responses measured with two common surround sound microphone setups in a concert and a lecture hall.

Modelling and order of acoustic transfer functions due to reflections from augmented objects

It is commonly accepted that the sound reflections from real physical objects are much more complicated than what usually is and can be modelled by room acoustics modelling software. The main reason for this limitation is the level of detail inherent in the physical object in terms of its geometrical and acoustic properties. In the present paper, the complexity of the sound reflections from a corridor wall is investigated by modelling the corresponding acoustic transfer functions at several receiver positions in front of the wall. The complexity for different wall configurations has been examined and the changes have been achieved by altering its acoustic image. The results show that for a homogenous flat wall, the complexity is significant and for a wall including various smaller objects, the complexity is highly dependent on the position of the receiver with respect to the objects.

Room volume estimation from diffuse field theory

Among the parameters relevant in room acoustics, the room volume is one of the most important. The general course in room acoustics research is to use the room volume in the prediction of room acoustic parameters such as reverberation time or total relative sound pressure level. Contrary to this, it has been investigated to what extent the room volume can be retrieved from a measured room impulse response. The approach followed is based on room acoustic diffuse field theory and requires correctly measured room impulse responses with the initial time delay corresponding to the source to receiver distance. A total of ten rooms of varying size and acoustic characteristics have been included. The results in three rooms were unreliable, which was explained by the particular acoustic characteristics. In the remaining rooms the results were numerically useful and consistent between different positions within the same room (relative standard deviation around 20%). The influence of source and receiver directivity ...

Multichannel Room Impulse Response Generation With Coherence Control

A method is proposed by which an arbitrary number of room impulse responses (RIRs) is generated from one input RIR. The method works by convolving the input RIR with a multitude of filters, one for each desired output RIR. The filters are designed in such a way that the coherence between all possible pairs of output RIRs feature assignable frequency-dependent values. An application example is given where the values for the coherence are given by the diffuse field coherence functions for the microphones in a five-element microphone array. The limitations in terms of achieving the desired coherence values exactly and avoiding spectral coloration are also discussed. The proposed method is most suitable for the part of the RIR that does not contain strong discrete reflections.

Acoustics of enclosed spaces: The differences when the dimensions are millimeters vs. tens of meters

The dimensions of the ear canal are at least 3 orders of magnitude smaller than those typically encountered in room acoustics but at the same time the range of wavelengths for audio applications is identical. This results in a disparity not only in length scale but also a disparity in time scale. The influence of these disparities on well-known room acoustics parameters or features such as the reflection density, the direct-to-reverberant ratio, the critical distance, or transfer function nulls is reviewed and highlighted. The nature of the two substantially different sound fields is also important for active sound control. Consequently, the respective relevance of total absorption as well as values of source and sink impedance are also compared.

Transducer modelling for optimal in-situ performance in a hearing instrument context

In a hearing instrument context the transducers design has to be optimised for optimal performance in the installed configuration. This is typically behind or inside the outer ear of the instruments wearer. Several examples will be shown where the in-situ acoustics is taken into account in the design or selection process of transducers. In this process acoustic modelling is used extensively. Transducers are modelled by network analogues and they are coupled under defined impedance conditions to 3D numerical acoustics models which incorporate the in-situ influence of e.g. the wearers ear anatomy. Moreover, in some cases the modelling also includes part of the signal processing present in the hearing instrument.

Objective sound field analysis based on the coherence estimated from two microphone signals

The coherence estimate function represents the coherence function for discrete-time and finite-length time signals. In order to avoid bias error in the estimation of the required spectral densities, there is always an averaging mechanism in either time, frequency or space involved. This averaging has important consequences in room acoustics because diffuse field equations are then applicable to reverberant fields. It will be shown how the coherence estimates from diffuse and reverberant fields differ as a function of the averaging constants. For reverberant fields it will further be shown that the dependence between coherence estimate and averaging constants is defined by the direct-to-reverberant energy ratio and the reverberation time. Finally, the existing analytical expression for the coherence estimate as a function of the direct-to-reverberant energy ratio is extended to several primary sources.

Simulation of insitu measurement of diffusion and scattering coefficients

Since it has become clear that simulation of the acoustical properties of a room is not successful unless also nonspecular reflections are taken into account, the determination of diffusion and scattering coefficients of boundary profiles is a hot topic in room acoustics research. Measurement of the impulse response of a boundary along a planar array of microphone positions enables us to calculate these coefficients in situ, avoiding the drawbacks of laboratory measurements (necessity of scale modeling, limited value of results for practical situations). Array technology makes it also possible to extrapolate the measured responses to the boundary surface, thus creating an acoustical image of the boundary. Next, scattering objects can be virtually added to this image and the impulse response of the modified configuration at the array position can be obtained by inverse extrapolation. This way, the effect of adding an object to a real environment is simulated in a highly realistic way. For several (virtual)...